2x convertor problems

[tpinchback]

I have a used 2x mark I teleconvertor that when put on my 70-200L 2.8 lens the focusing slows to almost not moving at 400mm, but when put at 140mm the focusing goes back to normal. What is going on? Please help.

thanks

[defordphoto]

I'll hazard a first guess: Dirty contacts.

[tpinchback]

I have cleaned the contacts using a pencil eraser and no luck

I also noticed the AF motor making a faint humming noise while the convertor is attached.

any other options before I ask this guy for a refund?